The present invention relates to turbine fan aircraft use. In particular, the present invention is directed toward a turbine fan driven by an Otto cycle engine such as a piston or rotary (e.g., Wankel) engine. The present invention makes possible the most flexible and effective installation of a ducted fan with a fixed horsepower source, namely a conventional Otto Cycle engine Effectiveness being defined as full utilization of the engine""s available horsepower at the chosen flight points. In a further embodiment of the present invention, a novel heat exchanger is provided which removes waste heat with minimal drag while boosting the fan system""s effective thermal efficiency by increasing the enthalpy of the working fluid. In yet another embodiment of the present invention bypass air from the turbine is used to supercharge the piston or rotary engine.
Gas turbine engines (e.g., Jet, fanjet, and the like) are known in the art for propelling jet aircraft. While powerful and efficient, they are very costly to purchase and maintain, placing them out of the reach of most individual aircraft owners, hobbyists, and homebuilders.
Ducted fans are also known in the art. Ducted fans are typically driven by piston engines or, in some instances, rotary (e.g., wankel) engines. A ducted fan typically comprises a propeller rotating within a duct housing. The duct reduces tip losses significantly. However, the drag created by the duct can be significant. As a result, ducted fans generally work best only at lower speeds.
Thus, a need exists in the art to provide a low-cost turbine type propulsion system which is affordable. The present inventor has experimented with driving turbines with Otto Cycle engines (e.g., piston engines and the like) with success. However, such a design presents problems when operating at altitude versus seal level.
If one uses a conventional fan of a given flow rate and pressure ratio coupled to a fixed horsepower source a problem arises as a result of changing air density with altitude. If the aircraft mission is to cruise as fast as possible, while using minimum power and fuel, climbing to high altitude is the most practical way to accomplish this. The thinner air at 35,000 feet produces much less drag than at sea level, and makes it possible to fly at high speed on quite limited horsepower.
The problem with the conventional fan/fixed horsepower source combination, is that if the designer optimizes the fan for high altitude cruise, there may be nowhere near enough horsepower to drive the fan at sea level (because of higher air density) and takeoff performance may be severely impaired. If on the other hand the designer optimizes the fan for takeoff, then cruise performance sufferers as the fan can""t ingest enough air to fully absorb the engines horsepower.
With the operation of conventional Otto Cycle engines, it is known that only about 25% of the heat energy produced by burning fuel actually goes to mechanical energy. Approximately 30% may be lost through the primary cooling system, 35% goes out the exhaust, and the remainder, about 15% may be radiated from the engine itself. The waste heat represents a potential resource for augmenting thrust, improving performance and reducing fuel consumption.
In a typical aircraft application using an Otto Cycle engine, provision must be made for removal of waste heat. This may be typically accomplished by an exhaust pipe vented to the atmosphere, and cooling air routed to a heat exchanger. While these methods have worked effectively for a long time they are not set up so as to benefit the aircraft, only getting rid of the waste heat.
Aircraft use less fuel at a given speed the higher they go, but their is a limit to how high they can go without the use of a turbo-supercharger. Adding a turbo-supercharger adds weight, cost, and heat to the engine compartment. A need exists, therefore, to provide such supercharging or other altitude compensation without the added weight and complexity of a supercharger or turbo-supercharger.
Water injection for boosting the output of internal combustion engines has been used for many years. During WWII, all the major participants used some version of this idea on their fighter aircraft. The water must be injected as a fine mist to get the optimum benefit. The finer the mist the better; the small droplets have much more surface area than do large droplets, and therefore absorb the heat energy in the combustion chamber more efficiently.
When the water droplets are subjected to the heat and pressure in the combustion chamber, their volume increases more than the volume increase from heating the same mass of air. This results in greater cylinder pressure for the same heat released in the combustion chamber, and more horsepower. There are other benefits too; the water droplets stabilize and smooth the flame front in the combustion chamber suppressing detonation. By suppressing detonation, the engine can operate at higher BMEP (brake mean effective pressure), which also equates to greater horsepower.
The only downside is a higher rate of acid build up in the oil, which is detrimental to the engines bearing surfaces. This happens without water injection, as part of the normal bi-product of combustion is water vapor which combines in the oil to form acids. An engine using water injection would have to have its oil changed more frequently.
Prior Art water injection systems typically employed an atomizer type misting sprayer to inject water droplets into the intake of an engine. However, such an atomizer sprayer may not provide consistent, uniform, and small enough droplets of water. A finer and more consistent mist may provide better performance for a water injection system.
The solution to the above mentioned problems, is to use what is termed herein a bimodal Fan. The fan is designed to be able to vary its flow rate and therefore its thrust and power requirement. Pressure ratio does not change. This is analogous to a variable pitch propeller which changes its pitch with decreasing air density and increasing airspeed to optimize performance. This is accomplished in the bimodal Fan by having two concentric but separate flow paths. The inner flow path may be fully open all the time and may be sized to provide best take off performance with the horsepower available. The second, outer flow path, concentric to the first, may be fully closed at take off, by a radial array of movable vanes or shutters.
In a second embodiment of the present invention, a low loss heat exchanger may be provided, the purpose of which is to transfer the waste heat of the Otto Cycle engine, (from the exhaust and cooling water), directly to the fan discharge air down stream of the fan. This serves two purposes: removal of he engine""s waste heat, and boosting the fan system""s effective thermal efficiency by increasing the enthalpy of the working fluid (fan discharge air). The heat exchanger may be constructed of a cluster of thin walled tubes which vary in cross sectional shape.
Using the primary fan to supercharge the engine provides an additional 6-8,000 feet of altitude capability, increasing cruising speed and lowering fuel consumption for a given speed and reducing engine wear. It also may be beneficial to a turbo-charged engine by either increasing its altitude capability over that offered by the turbo-charger alone, or reduce engine wear by letting the turbo-charger operate at a lower boost pressure than it would by itself but maintaining the same level of performance. The fan supercharger system is very simple: pressurized air from the inner fan flow path may be routed through hollow struts that inject it into a collector plenum which may be attached to the outside of the outer flow path shell.
In another embodiment of the present invention, an air mister nozzle uses high velocity air through an annular orifice. Instead of the classic perfume type mister (atomizer) which is an air jet intersecting a small water tube, this nozzle uses air from a small centrifugal blower or fan, to create the same bernoulli effect. The crucial difference is the geometry and construction of the nozzle itself. Air enters through the rear of the nozzle and passes through an array of holes into a plenum chamber.
The center bullet provides the contour for the narrow annular nozzle. The bullet is concentric to the nozzle body and fastened in place by a lock-nut. The narrow annular gap is on the order of 0.010 inches all the way around. The advantage of this, is that it provides the opportunity to precisely control the entry of the water into the high velocity flow stream. This is accomplished by a circumferential gap leading from an annular water chamber to the nozzle annulus.
Water is metered through a simple orifice (not shown) and brought to the nozzle by flexible tubing. The water enters the nozzle and fills the annular water chamber which feeds water to the circumferential gap introducing water into the airflow. As water emerges from the small gap, the shearing action of the high velocity air forms the small droplets desired for this application. This type of nozzle allows for even, efficient distribution of water with a limited amount of air mass flow, and at modest pressures.